All ungulates are mammals (Class Mammalia) - a group of warm-blooded
(endothermic) animals with backbones. Taxonomically, mammals are animals
(Kingdom Animalia) and belong to the Phylum Chordata, Subphylum Vertebrata.
Mammals can be distinguished from other animals by the presence of
milk-producing skin glands - these mammary glands, for which the Class Mammalia
is named, are found in every mammal and nowhere else in the Animal Kingdom.
Many other characters are used to diagnose mammals, including the presence
of hair, three middle ear bones, and a four-chambered heart.
Within the mammals, there are two extant subclasses: the Prototheria, containing
only three modern species of egg-laying monotremes (Order Monotremata); and
the Theria, or viviparous mammals (those which give birth to live young).
The Theria are subdivided further into the marsupials (infraclass
Metatheria), and the eutherians or placental mammals (Eutheria). Ungulates
- and most of the other 4,500+ mammal species - are eutherians, mammals which
nourish their young inside the mother's body using a chorioallantoic placenta
(in eutherians, the placenta acts as an endocrine organ during pregnancy).
The extended time which young can develop inside their mothers gives infant
eutherians a head-start in life, and is responsible for the ability of many
ungulates to run only hours after birth.
The two ungulate orders (Perissodactyla
and Cetartiodactyla) do not isolate
from the rest of the mammals together, and thus they do not form a distinct
taxonomic group. As we saw in What is an
Ungulate?, our understanding of these relationships is rather recent,
and is still making its way through the scientific literature. Because
the Ungulata is no longer valid, the true ungulates no longer stand alone,
but are instead nested within the infraclass Eutheria (and the superorder
Laurasiatheria). Since they cannot be keyed out further, each order
must be examined individually.
The easiest way to distinguish the two types of ungulates is to look at their
feet (and hooves): Perissodactyls (generally) have an odd number of toes
on each foot, while "artiodactyls" (the ungulates within Cetartiodactyla)
usually possess an even number of toes on each foot. Despite the even-odd
differentiation, there are exceptions to the rule: for instance, both tapirs
(Perissodactyla:
Tapiridae) and some peccaries
(Cetartiodactyla:
Tayassuidae) have four toes
(and hooves) on their front feet, and three on the rear. How is this
difference accounted for? It is not officially the NUMBER of toes,
but their ARRANGEMENT, that differentiates the two orders. Perissodactyls
have a mesaxonic foot structure, where the plane of symmetry of the foot
passes through the third digit (this digit also bears most of the animal's
weight). In artiodactyls, the third AND fourth digit are the principal
weight bearers (i.e., the axis of the foot passes BETWEEN the two toes),
with the result that their foot structure is paraxonic.
Find out more about each ungulate
order:
Perissodactyla AND Cetartiodactyla
Or keep exploring this site:
  
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Literature Cited
Martin, R. E., R. H. Pine, and A. F. DeBlase. 2001. A Manual
of Mammalogy, Third Edition. Boston: McGraw-Hill Publishing.
Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. 2000. Mammalogy,
Fourth Edition. Fort Worth: Saunders College Publishing:
Springer, M. S., M. J. Stanhope, O. Madsen, and W. W. de Jong. 2004.
Molecules consolidate the placental mammal tree. Treds in Ecology
and Evolution; 19(8): 430-438.
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